Residualizing labels are biologically inert, non-degradable radioactive or fluorescent tags for proteins which are retained in lysosomes following cellular uptake and catabolism of the carrier protein. Our laboratory has been involved in the development of these labels for a number of years, beginning with (3H)raffinose in 1978, and followed by dilactitol-125I-tyramine in 1985. These residualizing labels have proven useful in identifying the tissue and cell sites of catabolism of long-lived plasma proteins such as albumin, lipoproteins and immunoglobulins in vivo, as well as in studies on the uptake and catabolism of proteins by cells in culture. During the last two years we have prepared and characterized an improved radioactive label, inulin-125I-tyramine, and a new fluorescent label, N,N--dilactitol-N'-fluoresceinyl- ethylenedlamine, and have continued studies on the catabolism of albumin and IgM in vivo and on the catabolism of albumin by fibroblasts in culture. We have also made progress toward development of an 18F-label for use in noninvasive imaging by positron emission tomography. Our research goals for the next five year period are a continuation of our current effort to develop more useful labels, to use them in studies on the sites and mechanisms of plasma protein catabolism and to collaborate in the application of these labels for noninvasive and diagnostic imaging. Specific Aims are: 1. Continued work on fluorescent labels, to include development of a red fluorescent label and application of these labels in experiments in vivo and in vitro using fluorescence microscopy and fluorescence activated cell sorting; 2. Continued work on the synthesis and application of residualizing labels for non-invasive imaging, to include development of 18-F-labels suitable for positron emission tomography and 19F-Labels for magnetic resonance imaging, and application of these labels in studies on deposition of circulating lipoproteins, diagnostic imaging with anti-tumor antibodies and targeting of protein pharmaceuticals; and 3. Continued studies on the catabolism of serum proteins in vivo, to include (a) studies on the mechanisms involved in the regulation of albumin catabolism, with emphasis on the effects of fatty acid saturation on albumin catabolism, (b) studies on the sites of catabolism of IgG and IgM and the role of receptors in catabolism of these proteins, (c) studies on the catabolism of lactate dehydrogenase isozymes to determine the biochemical basis for differential kinetics of catabolism of the M4(short-lived) isozymes, and (d) studies on the catabolism of oxidized serum proteins.